Unit 11: How can we shift the balance of a reaction? (Rates of Reaction & Dynamic Equilibrium)

state of matter & kinetic theory

Matter exists in three physical states:

Solid (s) – particles are closely packed in fixed positions and vibrate due to strong intermolecular forces.
Liquid (l) – particles are close together but can move past one another because forces are weaker.
Gas (g) – particles are far apart, move rapidly in random directions, and collide with container walls, creating pressure.

According to the kinetic theory, all matter is made of tiny particles in constant random motion. The average kinetic energy of particles is directly proportional to temperature (in Kelvin).

Increasing temperature → particles move faster → more energetic collisions.
In solids particles vibrate only.
In liquids particles slide past each other.
In gases particles move freely and rapidly.

This theory explains diffusion, expansion of gases, and pressure in gas systems.

collision theory

A reaction occurs only when particles:

Collide
Collide with energy greater than or equal to the activation energy (Ea)
Collide with the correct orientation

Activation energy (Ea) is the minimum energy required for a successful collision.

If energy is too low, particles bounce apart and no reaction occurs.

On an energy profile diagram:

Ea is shown as the energy difference between the reactants and the peak.
The peak represents the activated complex.
The difference between reactants and products represents ΔH.

Increasing temperature increases:

Collision frequency
The proportion of particles with energy ≥ Ea

Therefore, reaction rate increases.

Energy Profile diagram:

rate of reaction

The rate of reaction is the change in concentration of a reactant or product per unit time.

Screenshot 2026-03-04 at 2.59.05 PM.png

Rate is highest at the start and decreases as reactants are used up.

The gradient of a concentration–time graph represents the rate:

Steeper gradient → faster reaction
Instantaneous rate is found using a tangent at a specific point

Factor	Explanation	Effect on Rate
Temperature	Increases particle kinetic energy → more collisions with ≥ activation energy	↑ Temperature → ↑ ROR
Concentration (solutions)	More particles per unit volume → collisions more frequent	↑ Concentration → ↑ ROR
Pressure (gases)	Compresses gas particles into smaller volume → increases collision frequency	↑ Pressure → ↑ ROR
Surface Area (solids)	Smaller particles expose more surface for collisions	↑ Surface area → ↑ ROR
Catalyst	Lowers activation energy by providing an alternative reaction pathway	Increases frequency of successful collisions

Data analysis involves:

Comparing gradients
Identifying initial rate
Comparing curves under different conditions